Nanoparticles in polymeric coatings may bring about targeted improvement in properties such as scratch resistance, UV protection, conductivity, etc. It is the control of surface modification and dispersing of the nanoparticles that determines the required transparent appearance of the coatings and also their properties. (Nanoparticle composites for coating applications. Cayton, Roger H. Editor(s): Laudon, Matthew; Romanowicz, Bart. NSTI Nanotech 2004, Boston, Mass., United States, Mar. 7-11, (2004), 3 312-315).
With regard to the introduction of the nanoparticles into coating or adhesive formulations there are different approaches that have been taken in the past. The particles can be mixed directly into the resin component or curative component or into the ready-to-apply coating compositions. In the case of aqueous systems there is the possibility of dispersing the particles into the aqueous phase. There have additionally been descriptions of the preparation of the particles in situ in one of the binder components, and of surface adaptation either to the resin or to the curative component.
From a practical viewpoint it is advantageous to disperse the nanoparticles in the form of stable masterbatches in one of the components, to ensure long-term storage stability and ease of handling at the stage of formulating, for example, coating materials or adhesives. In the end application the nanoparticles must likewise be dispersed well and finely, in order to produce advantageous properties such as transparency, scratch resistance, conductivity, etc.
In the art the nanoparticles are typically dispersed into the resin component, into the aqueous phase and/or into the completed mixture of curative and resin shortly prior to curing. This generally requires the surface of the nanoparticles to be adapted to the specific matrix of the coating composition or adhesive. A disadvantage of simply mixing modified nanoparticles in is the dependence of the stability on the complete formulation, i.e. on all the formulation constituents. Varying one parameter can lead here to separation (Pilotek, Steffen; Tabellion, Frank (2005), European Coatings Journal, 4, 170ff).
In “Polymers for Advanced Technologies (2005), 16(4), 328-331” a mixture of poly(tetramethylene glycol) and silicon dioxide produced by flame pyrolysis (nanosilica) is reacted with 4,4-diphenylmethane diisocyanate and the product is subjected to chain extension with 1,4-butanediol. The polyurethane chains were attached covalently via urethane bonds to the silica surfaces. Polyurethane films based on these approaches showed improved mechanical properties such as tensile strength and breaking elongation.
This kind of fumed silica is composed substantially of aggregates of sintered primary particles, and therefore, in comparison with silica in disperse form as primary particles, prepared wet-chemically, has a broader particle size distribution and a larger average particle size. In the case of fumed silica, these grave differences often lead to instances of inhomogeneity and turbidity even in the coating films and adhesive bonds that are obtainable from them. Furthermore, the covalent attachment of the particles to the polyurethane network may be critical for certain applications such as automotive clearcoating, since significant crosslinking raises the glass transition temperature and prevents elastic reflow. The reflow is of importance for the wet scratch resistance (Meier-Westhues, U. et al. (1999), Polyurethane clearcoats with optimized resistance to scratching and chemicals, Praxis-Forum, Meeting of the “Automobilkreis Spezial”, Bad Nauheim).
WO 2001005883 describes compact or cellular polyurethane elastomers with dispersed, non-agglomerated silica nanoparticles, and the process for preparation, the particles being able to be introduced into the polyol phase, into low molecular mass crosslinking agents or chain extenders and also, via inert solvent dispersions, into the isocyanate phase. No remarks are made in respect of film turbidity or gelling.
JP 002005162858 and JP 002005171017 describe hydrophilic polyurethane resins formed from polysiloxane, chain extender, polyisocyanate and high molecular mass polyols or polyamines. Organic silica sols are incorporated into such resins by dispersion, and the solvent is subsequently separated off. Features described as being advantageous in connection with this silica-modified resin include the high water adsorption and, in association therewith, anti-fogging properties, high transparency and flexibility, among others. Particle-modified polyisocyanates, however, are not disclosed.
In the first step of EP-A 372957 a prepolymer is prepared from isocyanate, polyol and amine, and excess NCO groups are blocked. The prepolymer, containing OH, NCO (blocked) and NH2 groups, is then blended with further formulating ingredients, including silica sol, and is applied as a primer and cured. Not described are the gelling stability and clouding stability of such silica-modified polyisocyanates.
JP 2005320522 describes the production of transparent hardcoat coatings for plastic films. NCO-containing and double-bond-containing polymers, silica sol in organic solvent, catalyst and acrylate component are mixed and applied to PET films, and cured thermally and via UV light.
JP 2004256753 likewise describes the use of nonmodified, colloidal silica containing silanol groups which is reacted with double-bond-containing isocyanates (2-methacryloyloxyethyl isocyanate). The Si—OH groups of the silica particles form a urethane bond as a result of reaction with NCO groups. The coating material actually crosslinks via polymerization of the acrylate groups. Disadvantageous features of these products are the low transparency of 88.2% and the high, uncontrolled degree of crosslinking, caused by the silanol-containing nanoparticle surfaces, which can be considered, for example, to be advantageous for the reflow characteristics and hence for the scratch resistance in the case of automotive coating materials.
DE-A 198 11 790 describes nanoparticle-containing, transparent film-forming binders into which fumed silicas and organically modified pyrogenic silicas have been incorporated into OH-functional binders by means of nozzle jet dispersion. Nozzle jet dispersion requires high shearing energies and is not a guarantee of complete nanoparticle dispersion. Sintered aggregates of fumed silicas cannot be reliably dispersed in primary particle form in that way and so may lead to instances of film clouding.
EP-A 1054046 adds microscale inorganic fillers to one or both binder components of an aqueous two-component PU formulation, the fillers specified including, preferably calcium carbonate or TiO2, sand, clay mineral, mica and dolomite.
Disadvantageous features of the methods stated above include the fact that the particles and particle agglomerates employed are too large to achieve such homogeneous dispersion in the binder components that clouding-free films can be produced even at high coat thicknesses. Likewise disadvantageous is the fact that the particles are used in very largely unmodified form, so that depending on the other binder components there may be phase separation and inhomogeneities in the ready-to-apply formulation. As a result of the relatively high crosslinking density, the covalent attachment of the particles to the polyurethane matrix leads to comparatively poor wet scratch resistance and to improvable reflow characteristics, and also to a buildup in viscosity, and often gelling, when the formulations are stored.
The use of surface-modified silica particles is described in DE-A 19540623, the particles being incorporated in a matrix material. Specified as matrix material are numerous polymeric materials, albeit none of the curatives such as epoxides or polyisocyanates that are typically employed in coating technology.
U.S. Pat. No. 6,022,919 discloses specific polyacrylate resins which are admixed together with silane-functionalized inorganic particles and are cured with isocyanates. The crosslinked films are notable for weathering stability plus resistance to effects caused by light or chemicals. To what extent such sols can be incorporated into the polyisocyanate component without increase in viscosity or gelling, on the other hand, is not apparent.